This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
A semiconductor superlattice is commonly made of two materials with different energy band gaps, in which each quantum well sets new selection rules that govern charge carrier distribution, dynamics, and transport. These structures are the foundation of many electronic and optoelectronic devices including light-emitting diodes, lasers, infrared photodetectors, transistors, thermoelectric devices, and solar cells. Conventional semiconductor quantum wells (i.g., mainly group III-V and IV compounds, e.g. GaAs/AlxGa1-xAs/GaAs) are grown in a layer-by-layer fashion under high temperature and high vacuum conditions. Their properties and performance are limited by interfacial lattice mismatch. Thus, the fabrication of defect-free quantum wells using new materials via innovative and cost-effective methods has been of great interest for a long time.
Two-dimensional (2D) semiconductor superlattices (or quantum wells), which are usually fabricated through metal-organic chemical vapor deposition or molecular beam epitaxy, are key building blocks in modern optoelectronics. The ability to simultaneously realize defect-free epitaxial growth and to individually fine-tune the chemical composition, layer thickness, and band structure of each layer is essential for achieving the desired device performance. Such structures are challenging to realize using organic or hybrid materials.
Organic semiconductors offer great structure and property tunability through synthetic manipulation of their molecular motifs. Moreover, their weak inter-molecular van der Waals interactions make them much more tolerant towards lattice mismatch. Quantum wells based on organic materials were initially investigated in the 1990s. See U.S. Pat. No. 6,420,056. However, key challenges remain to growing organic superlattices, including controlling the crystallinity and layer uniformity at the nanometer scale and reducing inter-diffusion at the organic-organic interface.
Two-dimensional (2D) layered halide perovskites have also attracted considerable attention. See U.S. Pat. No. 6,420,056. 2D perovskite structures can be understood as atomically thin slabs cut from the 3D parent structures along different crystal directions that are sandwiched by two layers of large organic cations. While there is a large number of reports regarding 2D perovskites that incorporate insulating aliphatic ammonium cations, only few efforts have been made to incorporate electronically-active organic moieties. However, in most attempts only polycrystalline thin films with low crystallinity were obtained. Common to all reports was that the incorporating of large conjugated organic groups into the inorganic matrix to make high quality superlattices was found to be challenging. To date, a fundamental understanding about how molecular structure influence the overall morphology and properties of the perovskites is still lacking, and the range of organic cations that can be incorporated into the lattice remains limited.
High-quality organic-inorganic hybrid perovskite quantum wells with tunable structures and band alignments are therefore still needed.